An antenna is the mechanism by which electromagnetic energy is radiated and received. The types and variations of antenna are extremely numerous, each type being of some particular advantage over the others for a specific requirement. Among the more important and frequently encountered requirements are those for operating bandwidth, directivity, both high and low, and polarization. Mounting factors for antennas, as in land vehicles, aircraft, and spacecraft, also pose problems of size, weight, air resistance, and vehicle interference.
For radar, although not necessarily for antennas in other electromagnetic applications, it is essential that the antenna enhance performance. A radar antenna has three basic roles: to be a major contribution to the radar's sensitivity, to provide the required surveillance and to allow measurements of angle of sufficient accuracy and precision.
One application where antenna configuration has been of considerable importance is airborne radar systems. Airborne radar systems, such as, for example, the radar in the U. S. Air Force's E-3A Airborne Warning and Control System (AWACS), use pulse-doppler radars; airborne pulse-doppler radars being more difficult to design and more complex than ground-based systems.
Performance of airborne radar of the early warning (EW) type have been limited by the physical constraints of the antenna. These constraints include range limitations as well as the inability to discriminate against targets of relatively small size.
Typical EW antennas are those utilized by the E-1, E-2, and E-3 aircraft, such antennas being dish antennas housed in a rotating dome situated above the fuselage of the aircraft by means of a pylon or the like. As the radome revolves, the emitted radar beam sweeps a circular band or ring around the airborne antenna platform. The height of the band is determined by the radar beam elevation angle, typically between approximately 15 and 20 degrees. Hence, at a range of about 300 nm, the radar beam will illuminate only those targets positioned within a band of approximately 75 to 100 nm high about the 360 degrees of a circle.
The radome antenna leaves blind spots above and below the aircraft that are not illuminated by the emitted radar beam, numerous airborne or surface threats possibly being located in these unilluminated areas. Further, the conventional radome antenna emits a single beam of electromagnetic energy having a finite width and height, the beam completing a single revolution on the order of once every 10 seconds. Therefore, the band swept by the beam is not continuously illuminated, but rather, is only illuminated intermittently. Another problem associated with known radome antennas is the creation of blind spots due to airframe interfernce.
The E-1 and E-2, being carrier-based aircraft, have antenna of relatively small size; however, have succeeded in achieving remarkable performance. The E-3, essentially a B-707 airframe, has a heavy dish antenna on the order of 11,000 pounds of relatively large size, on the order of 30 feet in diameter. Even with such a large antenna, the accuracy of known airborne radars tend to be worse than their ground-based counterparts by factors of ten.
Other approaches for mounting EW type antennas include conformal radar which utilizes wing structure as the vehicle for mounting of the antenna. However, many problems exist with this type of installation due to interference with the structure itself, engines/propellers, and other components of the aircraft.